The present invention relates to a process for checking the operatability of a nitrogen oxide storage catalyst which is used to remove the nitrogen oxides contained in the exhaust gas stream of a lean burn engine and contains at least a nitrogen oxide storage material, a catalytically active component and optionally an oxygen storage material, wherein the lean burn engine is operated with cyclic alternation of the air/fuel mixture from lean to rich and the nitrogen oxides contained in the exhaust gas are stored by the nitrogen oxide storage material in the presence of lean exhaust gas (storage phase) and desorbed and converted in the presence of rich exhaust gas (regeneration phase).
Nitrogen oxide storage catalysts were developed specifically for the treatment of exhaust gases from lean operated internal combustion engines. Diesel engines and lean burn gasoline engines belong to the group of lean operated internal combustion engines. Both types of engines are called lean burn engines in the following. Lean burn engines, in particular gasoline engines with a direct fuel injection system, are being used to an increasing extent in vehicle construction because they enable theoretical fuel savings of up to 25%, as compared with stoichiometrically operated internal combustion engines.
Nitrogen oxide storage catalysts have the ability to store nitrogen oxides over a wide temperature range under oxidizing exhaust gas conditions, that is during lean operation. This operating stage is therefore also called the storage phase in the following description.
Since the storage capacity of a storage catalyst is limited, it has to be regenerated from time to time. For this purpose, the air/fuel ratio in the air/fuel mixture supplied to the engine, and thus also the air/fuel ratio in the exhaust gas leaving the engine, is lowered to values of less than 1 for brief intervals. This process is also called enriching the air/fuel mixture of the exhaust gas. Thus, during this short operating phase, reducing conditions prevail in the exhaust gas prior to entry into the storage catalyst.
Under the reducing conditions present during the enrichment phase, the stored nitrogen oxides are released and reduced to nitrogen on the storage catalyst with simultaneous oxidation of carbon monoxide, hydrocarbons and hydrogen, as in the case of conventional three-way converters. This operating phase of the storage catalyst is also called the regeneration phase in the following. In the event of correct functioning of the total system consisting of storage catalyst, oxygen sensors and engine electronics, approximately stoichiometric conditions are present downstream of the storage catalyst during the regeneration phase, that is the hydrocarbons and carbon monoxide which are present in excess upstream of the storage catalyst during the regeneration phase are oxidized on the storage catalyst by the released nitrogen oxides. Only after completion of regeneration is there a sudden increase in reducing components downstream of the catalyst. This is called breakthrough of the reducing components through the storage catalyst.
The duration of the storage phase is typically about 30 to 100 seconds. The duration of the regeneration phase is substantially shorter and is in the region of only a few seconds (1 to 20 seconds).
The mode of operation and the composition of nitrogen oxide storage catalysts are known for example from EP 0 560 991 B1. As a storage material, these catalysts contain at least one component from the group of alkali metals (e.g. potassium, sodium, lithium, caesium), alkaline earth metals (e.g. barium, calcium) or rare earth metals (e.g. lanthanum, yttrium). As a catalytically active element, the storage catalyst contains platinum. Under oxidizing exhaust gas conditions, that is during lean operation, the storage materials can store the nitrogen oxides contained in the exhaust gas in the form of nitrates. For this purpose, however, the nitrogen oxides, about 60 to 95% of which consist of nitrogen monoxide, depending on the construction of the engine and its mode of operation, first have to be oxidized to nitrogen dioxide. This takes place on the platinum component of the storage catalyst.
In addition to the components mentioned above, the nitrogen oxide storage catalyst may also contain oxygen storing components. In this case, it can also take on the functions of a conventional three-way converter in addition to storing nitrogen oxides. Cerium oxide is mostly used as an oxygen storing component. The nitrogen oxide storage catalyst then has an oxygen storage function in addition to the nitrogen oxide storage function; thus it is bifunctional.
An important problem associated with modern exhaust gas treatment procedures is checking the correct functioning of the catalyst used in order to enable the timely replacement of catalysts which are no longer functioning efficiently. This also applies to nitrogen oxide storage catalysts, in which a variety of ageing mechanisms are observed. The nitrogen oxide storage capacity can be damaged on the one hand by the sulfur present in fuel and on the other hand by thermal stress. Whereas poisoning by sulfur can generally be counteracted by regenerating at elevated temperatures, thermal damage is an irreversible process.
In the case of bifunctional storage catalysts, in principle both storage functions (nitrogen oxide and oxygen) can be damaged by poisoning and by thermal effects. Damage to one function does not necessarily cause damage to the other function. Since nitrogen oxides and oxygen are both oxidizing components, their effects cannot be clearly separated from each other, so false diagnoses can be made when testing the catalyst.
DE 198 16 175 A1 discloses a process for checking the operatability of a nitrogen oxide storage catalyst which is intended to assess, separately, the oxygen storage function and nitrogen oxide storage function of the catalyst. To check the operatability of the storage catalyst in accordance with this document, the air/fuel ratio of the exhaust gas is switched from lean to rich and the time interval xcex94t1 obtained between the first change over up to breakthrough of the rich exhaust gas through the catalyst and also the time interval xcex94t2 obtained after switching the exhaust gas back from rich to lean, between the second change over up to breakthrough of oxygen through the catalyst, are measured. The time differences xcex94t1 and xcex94t2 permit separate assessment of the oxygen storage function and the nitrogen oxide storage function of the catalyst.
The nitrogen oxide storage function of the catalyst depends on the nitrogen oxide storage material and on the catalytically active component, generally platinum. Both the nitrogen oxide storage material and the catalytically active component may be damaged.
The nitrogen oxide storage material stores the sulfur dioxide contained in the exhaust gas in the form of sulfates. This takes place at the expense of the nitrogen oxide storage capacity. The sulfates in the storage material are substantially more stable than the nitrates. However, they can be decomposed again at exhaust gas temperatures higher than 600xc2x0 C. and under reducing conditions. As a result of this desulfurization process, the nitrogen oxide storage material largely regains its original nitrogen oxide storage capacity.
The nitrogen oxide storage capacity of the storage material depends critically on the specific surface area with which it can interact with the exhaust gas. If the storage material is subjected to exhaust gas temperatures higher than about 800xc2x0 C., the specific surface area becomes irreversibly reduced and its nitrogen oxide storage capacity decreases.
For optimum use of the catalytically active component, it is applied to the oxidic material of the storage catalyst in a highly dispersed form with average particle sizes between about 2 and 15 nm. Due to their very fine distribution, the platinum particles have a large surface area for interacting with the constituents in the exhaust gas. In particular in the lean exhaust gas of lean burn engines, irreversible enlargement, for example, of the platinum crystals is observed with increasing exhaust gas temperature and this is accompanied by an irreversible reduction in catalytic activity.
Using the process according to DE 198 16 175 A1 possible damage to the nitrogen oxide storage material and also to the catalytically active component are determined simultaneously. Assessment of damage to the storage material separately from damage to the catalytically active component is not possible using this process. Separate assessment of the catalytically active component is desirable, however, because a storage catalyst in which the component which is catalytically active towards the oxidation of nitrogen monoxide has been thermally damaged still has sufficient activity for exhaust gas treatment under stoichiometric exhaust gas conditions.
Thus, an object of the present invention is to provide a process which, in addition to assessing the storage materials in the storage catalyst, is also able to detect possible damage to the catalytically active component in the storage catalyst.
The above and other objects can be achieved by a process according to the invention for checking the operatability of a nitrogen oxide storage catalyst which is used for removing the nitrogen oxides contained in the exhaust gas stream of a lean burn engine and contains at least a nitrogen oxide storage material, a catalytically active component and optionally an oxygen storage material. According to the invention the lean burn engine is operated with cyclic alternation of the air/fuel mixture from lean to rich and the nitrogen oxides contained in the exhaust gas are stored by the nitrogen oxide storage material in the presence of lean exhaust gas (storage phase) and are desorbed and converted in the presence of rich exhaust gas (regeneration phase).
The process is characterized in that, to determine possible damage to the catalytically active component, the nitrogen oxide storage capacity of the nitrogen oxide storage catalyst is determined at exhaust gas temperatures which are within the range in which the oxidation of nitrogen monoxide to nitrogen dioxide is kinetically controlled, whereas no change in nitrogen oxide capacity is observed in the thermodynamically controlled range and, to determine possible damage to the storage material, the nitrogen oxide storage capacity of the nitrogen oxide storage catalyst is determined at exhaust gas temperatures which are within the range in which the oxidation of nitrogen monoxide to nitrogen dioxide is thermodynamically controlled.
When, in the context of this invention, the components in the storage catalyst are referred to in the singular, this is done in order to make the discussion easier to understand. A person skilled in the art will obviously understand that, in order to optimize the properties of the storage catalyst, different nitrogen oxide storage materials and also oxygen storage materials and several catalytically active components (for example, platinum, palladium, rhodium, iridium) may be combined with each other.
The process according to the invention is based on the ideas described below relating to the mode of action of nitrogen oxide storage catalysts.
According to the acknowledged theories of nitrogen oxide storage catalysts, the nitrogen oxides in the exhaust gas are bonded to the storage materials in the catalyst in the form of nitrates. However, only nitrogen dioxide reacts with the storage material to form the corresponding nitrates. Since about 60 to 95 vol. % of the nitrogen oxides in the exhaust gas from an internal combustion engine consists of nitrogen monoxide, this must first be oxidized to nitrogen dioxide on the catalytically active component in the storage catalyst before it can react with the storage material to form nitrates.
The nitrogen oxide storage capacity of a storage catalyst is thus determined by two processes:
a) oxidation of nitrogen monoxide on the catalytically active component in the storage catalyst to give nitrogen dioxide in accordance with the following equilibrium reaction equation:
2NO+O22NO2xe2x80x83xe2x80x83(1) 
b) reaction of nitrogen dioxide with the storage material to form the corresponding nitrates.
During the oxidation of nitrogen monoxide in an oxygen-containing atmosphere on the catalytically active component in accordance with reaction equation (1), a specific equilibrium is set up between nitrogen monoxide and oxygen on the one side and nitrogen dioxide on the other side, this depending on the particular temperature. Above a temperature of about 300xc2x0 C., the equilibrium being set up corresponds to the thermodynamic equilibrium, that is the equilibrium is shifted to the left with increasing temperature due to the highly exothermic nature of the formation of nitrogen dioxide from nitrogen monoxide and oxygen. Above about 650xc2x0 C., nitrogen monoxide reacts with oxygen to form nitrogen dioxide to only a very small extent. Below 300xc2x0 C. the thermodynamic equilibrium in accordance with reaction equation (1) shifts to the right with decreasing temperature, that is the formation of nitrogen dioxide is thermodynamically favoured in this temperature range. Nevertheless, the formation of nitrogen dioxide decreases again below about 300xc2x0 C. due to kinetic restrictions and is negligible below 100xc2x0 C. The temperature limit of about 300xc2x0 C. mentioned here applies to an oxygen content in the gas mixture of 6 vol. %.
When performing the process in a practical situation, measurements are made in the kinetically controlled range below 300xc2x0 C. and in the thermodynamically controlled range, preferably between 350 and 450xc2x0 C.
The nitrogen oxide storage capacity of the catalyst in the kinetically controlled temperature range is strongly dependent on the available surface area of the platinum particles. The larger the platinum surface area, the smaller is the effect of kinetic restrictions on the formation of the thermodynamically favoured nitrogen dioxide. In contrast, the platinum surface area plays only a much smaller part in the thermodynamically controlled range for the formation of nitrogen dioxide. The critical factor for nitrogen oxide storage capacity in this range is rapid removal of the nitrogen dioxide formed from the equilibrium in accordance with reaction equation (1), as a result of reaction with the storage material to form nitrates, which then leads to fresh production of nitrogen dioxide in accordance with the equilibrium. The formation of nitrate is dependent on possible damage to the storage material by thermal effects or by sulfur poisoning.
Thus, by checking the nitrogen oxide storage capacity of the catalyst in both temperature ranges (kinetically controlled range and thermodynamically controlled range for the formation of nitrogen dioxide) it is possible to differentiate damage to the catalytically active component from damage to the storage material.